1. Field of the Invention
The present invention relates to a solid golf ball including a core and cover, and, more particularly, to a solid golf ball which has improved flight performance and durability against cracking and which provides a golfer with good feeling on impact.
2. Description of the Related Art
Since solid golf balls are more excellent than thread-wound golf balls in terms of flight performance and durability against cracking, solid golf balls are widely used among amateur golfers. However, solid golf balls having higher durability against cracking have been demanded.
One conceivable measure for coping with such demand is increasing the hardness or thickness of the cover of a solid golf ball to thereby improve durability against cracking. However, conventional techniques for increasing the hardness or thickness of the cover of a solid golf ball have involved various problems.
For example, Japanese Patent Publication No. JP-A-93-123422 discloses a golf ball having a hard cover of a thickness of 2.2 to 2.9 mm; Japanese Patent Publication No. JP-A-94-154357 discloses a golf ball having a hard cover of a thickness of 2.1 to 2.8 mm; and Japanese Patent Publication No. JP-A-97-28830 discloses a golf ball having a hard cover of a thickness of 3 to 5 mm. However, in these golf balls, since the cover is excessively hard, durability against cracking and feeling on impact are not satisfactory.
Further, Japanese Patent Publication No. JP-A-97-28831 discloses a golf ball having a cover of a thickness of 3 to 5 mm and a Shore D hardness of 50 to 60. However, this golf ball does not have sufficient flight performance, because the cover is made excessively soft in order to improve controllability and feeling.
Moreover, Japanese Patent Publication No. JP-A-96-276033 discloses a golf ball designed such that the difference in p-hardness between the core and the entirety of the golf ball falls within a predetermined range in order to improve flight performance and feeling. However, this golf ball does not have sufficient flight performance, because of a flat hardness distribution of the core.
In view of the foregoing, an object of the present invention is to provide a solid golf ball which, through increasing the thickness of the cover, has improved flight performance and durability against cracking, while providing a golfer with good feeling on impact.
To achieve the above object, the present invention provides a solid golf ball comprising a core and a cover, the golf ball satisfying the following conditions (1) to (3):
(1) the cover has a thickness of at least 2.5 mm;
(2) the cover has a Shore D hardness of 59 to 62; and
(3) a value obtained through subtraction of JIS C hardness of a center portion of the core from JIS C hardness of a surface portion of the core is at least 8.
In the golf ball of the present invention, the thickness and hardness of the cover are determined in consideration of attaining balance therebetween, and the difference between the JIS C hardness of the core center portion and that of the core surface portion is set optimally. Thus, the solid golf ball of the present invention has increased durability against cracking and enhanced flight performance and provides a golfer with improved feeling on impact.
The present invention will next be described in detail. The solid golf ball of the present invention includes a core and a cover. The core material is not particularly limited. For example, vulcanized rubber containing a predominant amount of polybutadiene rubber, polyisoprene rubber, natural rubber, or silicone rubber can be used. However, vulcanized rubber containing a predominant amount of polybutadiene rubber is particularly preferred.
In the present invention, pentachlorothiophenol or a metallic salt of pentachlorothiophenol may be added to the core material, which enables formation of a core having high resilience, to thereby improve the flight performance of the ball. Pentachlorothiophenol or a metallic salt of pentachlorothiophenol is preferably a zinc salt of pentachlorothiophenol; for example, Renacit IV (product of Bayer Corp.). Pentachlorothiophenol or a metallic salt thereof is added to the core material in an amount of 0.1 to 2.0 parts by weight, preferably 0.2 to 1.0 parts by weight, on the basis of 100 parts by weight of the base rubber. When the incorporation amount of pentachlorothiophenol or a metallic salt thereof is excessively small, the resilience of the core cannot be increased sufficiently, so that the resilience of the golf ball may decrease with a resultant decrease in travel distance. When the incorporation amount of pentachlorothiophenol or a metallic salt thereof is excessively large, the pentachlorothiophenol or a metallic salt thereof hinders cross linking reaction of a rubber composition that forms the core.
In the present invention, the value obtained through subtraction of JIS C hardness of a center portion of the core from JIS C hardness of a surface portion of the core is at least 8, preferably 8 to 20, more preferably 10 to 18. When the hardness difference is excessively small, spin of the golf ball upon hit by a driver increases excessively, possibly resulting in a decrease in travel distance. When the hardness difference is excessively large, durability against cracking may be impaired.
The JIS C hardness of the core surface portion is typically 70 to 85, preferably 74 to 81. When the JIS C hardness of the core surface portion is excessively low, spin of the golf ball increases, possibly resulting in a decrease in travel distance. When the JIS C hardness of the core surface portion is excessively high, feeling on impact may become too hard, and durability against cracking may be impaired.
The JIS C hardness of the core center portion is typically 55 to 69, preferably 57 to 67, more preferably 59 to 65. When the JIS C hardness of the core center portion is excessively low, durability against cracking may be impaired. When the JIS C hardness of the core center portion is excessively high, feeling on impact may become too hard, and spin of the golf ball increases, possibly resulting in a decrease in travel distance.
The xcexc-hardness of the core; i.e., deflection (deformation) amount when applied load is increased from an initial load of 98N (10 kgf) to 1275 N (130 kgf), is typically 2.7 to 5.5 mm, preferably 3.0 to 5.0 mm, more preferably 3.3 to 4.5 mm. When the xcexc-hardness of the core is excessively high, feeling on impact may become too hard, and spin of the golf ball increases, possibly resulting in a decrease in travel distance. When the xcexc-hardness of the core is excessively low, feeling on impact may become too soft, and durability against cracking may be impaired.
The core may assume a single-layer structure formed of a single material or a multilayer structure composed of two or more layers of different materials. When the core has a multilayer structure, xe2x80x9cJIS C hardness of a surface portion of a corexe2x80x9d appearing previously means the hardness of a surface portion of the outermost layer of the core. When the core has a multilayer structure, preferably, all the layers of the core are mainly formed of a rubber material.
A cover material of the golf ball of the present invention is not particularly limited. For example, the cover may be made of an ionomer resin or a mixture of a predominant amount of an ionomer resin and a polyester resin, a polyurethane resin, a polyamide resin, or a polyolefin resin. Particularly, ionomer resins such as those commercially available from Du Pont, Ltd. under the trade name Surlyn and from Du Pont Mitsui Polychemicals Co., Ltd. under the trade name Himilan are preferred.
The material suitable for the cover of the golf ball of the present invention contains a dominant amount of a blend resin containing an ionomer resin neutralized by divalent metal ions (e.g., Zn++) (10 to 90% by mass) and an ionomer resin neutralized by monovalent metal ions (e.g., Na+) (90 to 10% by mass). This enables formation of a cover having high durability and sufficient resilience. The ionomer resin neutralized by divalent metal ions and the ionomer resin neutralized by monovalent metal ions are preferably blended together at a ratio (20 to 80% by mass): (80 to 20% by mass), more preferably, (25 to 75% by mass): (75 to 25% by mass).
The cover has a thickness of at least 2.5 mm, preferably, at least 2.7 mm, more preferably, at least 3.0 mm. When the cover is excessively thin, durability against cracking becomes insufficient. Further, the thickness of the cover is not greater than 5 mm, preferably not greater than 4 mm. When the cover is excessively thick, the resilience of the golf ball decreases, possibly resulting in a decrease in travel distance.
The cover has a Shore D hardness of 59 to 62, preferably, 59 to 61, more preferably, 60 to 61. When the cove is excessively soft, the resilience of the golf ball decreases, and spin of the golf ball increase, possibly resulting in a decrease in travel distance. When the cover is excessively hard, durability against cracking is impaired, as is feeling upon hit by a club, in particular, an iron club.
The thus-obtained golf ball may be finished through application of coating to the surface and stamping the surface with marking as needed. The xcexc-hardness of the entire ball; i.e., deflection (deformation) amount when applied load is increased from an initial load of 98N (10 kgf) to 1275 N (130 kgf), is preferably 2.3 mm to 3.5 mm, more preferably 2.6 mm to 3.3 mm. The ball can be formed in such a manner as to have a diameter not less than 42.67 mm and a weight not greater than 45.93 g as specified under the Rules of Golf as approved by RandA.